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Pericyclic Reactions: Sigmatropic Rearrangements

Sigmatropic Rearrangements: Examples for [1,n] Sigmatropic 6- and 8-Electron Systems

6-Electron systems, [1,5] sigmatropic

The parent system reacts at approximately 250°C with an activation energy barrier of 36 - 38 kcal/mol.

Parent system of a [1,5] sigmatropic H-shift

The [1,5] hydrogen shift in cyclopentadien can be obserevd already at roomtemperature. At 60°C, migration is so fast that only one signal appears for all hydrogens in 1H-NMR.

[1,5] H-Shift in cyclopentadiene

2D Animation of the sigmatropic hydrogen shift in cyclopentadiene.

The situation becomes more complicated when different conformers of the same compound react. In order to test the Woodward-Hoffmann rules, Roth synthesized a chiral 1,3-pentadiene system containing different substituents:

Test molecule for the Woodward-Hoffmann rules

Only compounds with (E) configuration at the terminal double bond and (S) stereochemistry at the terminal sp3 stereocenter together with the corresponding (Z),(R)-isomer were found. Antarafacial hydrogen shift was not observed.

[1,5] sigmatropic migrations of alkyl residues (suprafacial, retention) are relatively rare.

[1,5] Sigmatropic alkyl shift

2D Animation of [1,5] sigmatropic alkyl migration with retention

It is known that 7,7-substituted cycloheptatrienes isomerize via a cascade of electrocyclic and [1,5] sigmatropic rearrangements.

8-Electron systems ([1,7] sigmatropic)

Antarafacial [1,7] sigmatropic rearrangements are only possible by twisting the reacting system. The best known example is the formation of vitamin D3 in the skin from dehydrocholesterol by UV irradiation. The photochemical electrocyclic ring opening is followed by a spontaneous [1,7] sigmatropic hydrogen shift.

[1,7] Sigmatropic H-shift in vitamin D3 synthesis

The twisted conformation already exists in the steroid framework.

Starting geometry for the antarafacial rearrangement
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